1. Field of the Invention
The present invention relates to methods for joining a sintered hard alloy member to a supporting member made of a softer metal. More particularly, the invention relates to a method of joining a member such as tungsten carbon alloy rod to a member such as a stainless steel shank, and the method of making a cutting tool such as a twist drill bit from the joined members.
2. Description of Background Art
Sintered hard alloys whose main component is tungsten carbide and which include cobalt, tantalum, chrome or the like, have exceptionally high hardness, and are routinely used as materials from which cutting tools such as twist drills are manufactured. Thus, for example, twist drill bits, especially those used for drilling hard or abrasive materials such as glass-filled printed circuit boards, are manufactured from a sintered hard alloy blank having the shape of a rod which is machined to form a cylindrical rear shank portion adapted to be clamped in the collet of a drill, and a smaller diameter front rod-shaped portion in which is machined a conical cutting point and helically disposed cutting edges.
Drill bits in which both the cutting part and shank part of the bit are made of a single piece of relatively expensive sintered hard alloy have a relatively high raw materials cost. Moreover, since the diameter of the front cutting portion of most drill bits is smaller than that of the rear shank portion, substantial machining time is required to reduce the diameter of the cutting part to a reduced size, and sintered hard alloy material removed during the machining process is wasted.
Because of the relatively higher cost and brittleness of sintered hard alloys compared to other metals such as stainless steel, some cutting tools such as certain drill bits are manufactured as a composite structure, in which a cutting members or surfaces thereof are made of a sintered hard alloy, such as tungsten carbide, and a supporting structure for the hard alloy cutting member which is generally made of a softer, less expensive, more readily machinable material such as stainless steel.
In an example of a prior art composite drill bit structure, a cylindrical cavity is formed in the front end face of a stainless steel rod, and a sintered hard alloy rod is shrink-fitted into the cavity. Cutting surfaces are then machined in the hard alloy rod to form a completed twist drill.
Drill bits made as composite structures as described in the foregoing example are more economical to manufacture than drill bits made entirely of a sintered hard alloy material. However, shrink fitting a hard alloy rod into a cavity within a stainless steel shank rod requires very accurate machining of the cavity. Also, the length of the sintered hard alloy material inserted into the shank cavity adds to the length required of the sintered hard alloy beyond that required for cutting surfaces, thus adding to the cost of the composite drill bits manufactured by this method.
The present invention was conceived of to provide an economical method of joining a sintered hard alloy member to a stainless steel member to thereby make a composite structure in which the two members are firmly bonded to one another, and a method of making a cutting tool from the composite structure.
An object of the present invention is to provide a method of joining a sintered hard alloy member to a softer metal supporting member.
Another object of the invention is to provide a method of joining a sintered hard alloy member to a stainless steel member.
Another object of the invention is to provide a method of joining a carbide alloy member to a stainless steel member.
Another object of the invention is to provide a method of joining a sintered hard alloy member to a softer metal member, which does not require forming a cavity in the softer member.
Another object of the invention is to provide a method of joining a sintered hard alloy member to a softer metal member having a circular end face.
Another object of the invention is to provide a method of joining a sintered hard alloy member to a softer metal member having a conically-shaped end face.
Another object of the invention is to provide a method of joining a sintered hard alloy rod to a softer metal shank having a transverse end face, the method including the steps of exerting a longitudinally disposed force on the rod relative to the shank to thereby exert pressure on a contact region between the shank and rod end faces, and applying a pulse of electrical current through the contact region sufficient to cause melting and melding of material from the rod and shank at the junction between them.
Another object of the invention is to provide a method of joining a sintered, hard alloy rod to a softer metal shank having a conically-shaped transverse end face, the method including the steps of exerting a longitudinally disposed force on the rod relative to the shank to thereby exert pressure on a contact region between the conical shank end face and rod end face, and applying a pulse of electrical current through the contact region sufficient to cause melting and melding of material from the rod and shank at the junction between them.
Another object of the invention is to provide a method of manufacturing a cutting tool structure including the steps of forcing an end face of a hard alloy rod member against the end face of a shank made of a softer metal, passing a pulse of electrical current through a contact region between the shank and rod member end faces sufficient to melt and meld the shank and rod member together at the contact region, and forming a cutting surface on the hard alloy rod member.
Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims.
It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, we do not intend that the scope of our exclusive rights and privileges in the invention be limited to details of the embodiments described. We do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims.
Briefly stated, the present invention comprehends a method of joining a sintered hard alloy member such as a drill bit rod blank made of a material such as tungsten carbide, to a softer metal member such as a stainless steel shank to thereby form a strong composite structure and of machining cutting surfaces on the hard alloy member part of the composite structure to make a cutting tool.
According to the invention, an end face, e.g., the front end face of a stainless steel shank and an end face of a hard alloy rod are pressed against each other at a contact region, and a pulse of electrical current passed between the rod and shank through the contact region. The current pulse is sufficiently energetic to cause localized melting of both hard alloy member and shank end faces, thus causing molecules of both members to intermingle, meld, and form a diffusion bonded weld upon cooling. As a result of this process, a hard alloy carbide drill bit rod blank and a stainless steel shank, for example, are welded together with a bonding strength that is as great as the fracture strength of the rod.
In a preferred embodiment of the invention, a convex conical end surface is formed in a transverse end face of a stainless steel shank, thus centrally localizing pressure and heating during the aforementioned welding process.
According to the invention, cutting edges are machined in the hard alloy rod after it has been welded to the shank, to form a finished drill bit.